1. Field of the Invention
The present invention relates to an optical fiber comprising a structure which can yield a favorable characteristic when formed with a grating whose refractive index changes along an advancing direction of light, and an optical component such as an optical fiber coupler constituted by the optical fiber.
2. Related Background Art
An optical fiber coupler is an optical component in which respective predetermined parts (optical coupling portions) of a plurality of optical fibers are optically coupled, and has functions of multiplexing, demultiplexing, and branching out light, for example. For instance, each of Satoda, et al., xe2x80x9cAdd-Drop Multiplexer Using Optical Fiber Grating Coupler,xe2x80x9d Advance Proceedings of the 2000 IEICE Spring Conference, Lecture No. C-3-65 (literature 1); Igarashi, et al., xe2x80x9cFabrication of Fused Optical Fiber Grating Coupler,xe2x80x9d Advance Proceedings of the 1999 Fall JSAP Conference, Lecture No. 2p-ZF-8 (literature 2); and Igarashi, et al., xe2x80x9cImprovement of Fused Optical Fiber Grating Coupler,xe2x80x9d Advance Proceedings of the 2000 Spring JSAP Conference, Lecture No. 30a-ZG-18 (literature 3) discloses an optical component (optical fiber grating coupler) having an optical coupling part formed with a grating whose refractive index changes periodically.
FIG. 1 is a view showing the configuration of an optical fiber grating coupler as an optical component. The optical fiber grating coupler 1 shown in this drawing comprises a first optical fiber 11 and a second optical fiber 12 which are optically coupled to each other at a predetermined part (optical coupling portion) 13. The optical coupling portion 13 is provided with a Bragg grating whose refractive index changes along a signal direction of light. The optical fiber grating coupler 1 is manufactured by the steps of preparing the first and second optical fibers 11, 12 each having a core region doped with GeO2; arranging the first and second optical fibers 11, 12 closely in contact with each other in parallel and fusing their predetermined parts together while extending them, so as to form the optical coupling portion 13; and irradiating the optical coupling portion 13 with ultraviolet light by a phase grating method or two-beam interference method so as to form a grating.
In the optical fiber grating coupler 1, light entering from a first end 11a of the first optical fiber 11 reaches the optical coupling portion 13. Here, if light having a wavelength satisfying the Bragg condition of the grating in the optical coupling portion 13 reaches the optical coupling portion 13, the light is reflected by the optical coupling portion 13, so as to be emitted from a first end 12a of the second optical fiber 12. If light having a wavelength not satisfying the Bragg condition of the optical coupling portion 13 reaches the optical coupling portion 13 from the first end 11a of the first optical fiber 11, then the light having reached the optical coupling portion 13 is transmitted through the optical coupling portion 13, so as to be emitted from a second end 12b of the second optical fiber 12. In this case, the optical fiber grating coupler 1 functions as a demultiplexer by which the light entering from the first end 11a of the first optical fiber 11 is emitted to one of the first end 12a and second end 12b of the second optical fiber 12 depending on its wavelength.
In the optical fiber grating coupler 1, light entering from the first end 11a of the first optical fiber 11 reaches the optical coupling portion 13. Here, if light having a wavelength not satisfying the Bragg condition of the grating provided in the optical coupling portion 13 reaches the optical coupling portion 13, then the light having reached the optical coupling portion 13 is transmitted through the optical coupling portion 13, so as to be emitted from the second end 12b of the second optical fiber 12. On the other hand, light entering from a second end 11b of the first optical fiber 11 also reaches the optical coupling portion 13. Here, if light having a wavelength not satisfying the Bragg condition of the grating provided in the optical coupling portion 13 reaches the optical coupling portion 13, then the light is reflected by the optical coupling portion 13, so as to be emitted from the second end 12b of the second optical fiber 12. In this case, the optical fiber grating coupler 1 functions as a multiplexer by which the light entering from the first end 11a of the first optical fiber 11 (light having a wavelength not satisfying the Bragg condition) and the light entering from the second end 11b of the first optical fiber 11 (light having a wavelength satisfying the Bragg condition) are multiplexed, and thus multiplexed light is emitted from the second end 12b of the second optical fiber 12.
Since the optical fiber grating coupler 1 has the respective functions of the demultiplexer and multiplexer, it also functions as an optical ADM (add-drop multiplexer). An optical component (optical fiber coupler) whose optical coupling portion 13 is not provided with a grating functions as a demultiplexer by which light entering from the first end 11a of the first optical fiber 11 is branched out so that one of thus branched light components is emitted from the second end 11b of the first optical fiber 11 whereas the other of the branched light components is emitted from the second end 12b of the second optical fiber 12.
The inventors studied the conventional techniques mentioned above and, as a result, have found problems as follows. The grating provided at the optical coupling portion 13 in the optical fiber grating coupler 1 acting as the optical component is required to function so as to reflect light having a wavelength satisfying the Bragg condition at a high reflectivity and transmit light having a wavelength not satisfying the Bragg condition at a high transmissivity. Therefore, as suggested in F. Bakhti, et al., xe2x80x9cOptical add/drop multiplexer based on UV-written Bragg grating in a fused 100% coupler,xe2x80x9d Electronics Letters, Vol. 33, No. 9, pp. 803-804 (1997) (literature 4), and the above-mentioned literatures 2 and 3, for example, adding GeO2 not only to the respective core regions of the first and second optical fibers 11, 12 but also throughout their cladding regions so as to form a grating not only in the core region but also throughout the cladding region in the optical coupling portion 13 has been under consideration.
However, the inventors have found that the following problems occur if GeO2 is contained not only in the core region of each optical fiber but also throughout the cladding region thereof. Namely, strength extremely decreases in an optical fiber in which the whole cladding region including the core region is doped with GeO2. When making the optical fiber grating coupler 1 such as the one mentioned above by using this optical fiber, GeO2 is likely to evaporate and diffuse upon the fusion for forming the optical coupling portion 13, thereby generating bubbles in the optical fibers. As a consequence, there has been a problem that the fusion becomes difficult or the fusing strength weakens.
In order to overcome the above-mentioned problems according to the inventors"" findings, it is an object of the present invention to provide an optical fiber having a sufficient strength, in which not only a core region but also a cladding region is doped with GeO2; and an optical component including the same and having excellent optical coupling characteristics, which is easy to manufacture.
The present invention provides an optical fiber, mainly composed of silica glass, having an outer diameter D; the optical fiber comprising, at least, a core region extending along a predetermined axis, a first cladding region disposed at an outer periphery of the core region, and a second cladding region disposed at an outer periphery of the first cladding region. Each of the core region and the first cladding region is doped with 1 wt % or more of GeO2. On the other hand, the GeO2 content in the second cladding region is less than 0.2 wt %. The first cladding region has an outer diameter of 0.80D or more but 0.98D or less.
Since the optical fiber comprises the structure mentioned above, it yields a high strength. Also, since the GeO2-containing region extends over both of the core region and first cladding region, a grating can be formed throughout both of these regions, whereby excellent optical coupling characteristics can be obtained.
The optical fiber according to the present invention may be configured such that the GeO2 content in the first cladding region is 20% to 180% of that in the core region. In this case, the transmission loss occurring on the shorter wavelength side from the reflection wavelength in an area formed with the grating is lowered to a practically unproblematic level.
The optical fiber according to the present invention may be configured such that the first cladding region has a refractive index higher than that of the second cladding region, whereby the first cladding region has a relative refractive index difference of 0.01% or more with reference to the refractive index of the second cladding region. In an optical component such as an optical fiber grating coupler employing the optical fiber in this case, the quantity of light leaking from the optical coupling portion to the outside is suppressed low, and stable optical coupling characteristics are obtained.
In the optical fiber according to the present invention, each of the first and second cladding regions may be doped with additives which lower the refractive index of silica glass, such as F element and B2O3, for example. In this case, even when differences in GeO2 content alone cannot fully yield differences in refractive index between the core region, first cladding region, and second cladding region, sufficient refractive index differences can be secured (a desirable refractive index profile can be realized) between these regions if the respective F element contents in first and second cladding regions are set appropriately.
The present invention provides an optical component comprising first and second optical fibers each having the same structure as that of the optical fiber mentioned above (the optical fiber according to the present invention), in which the first and second optical fibers are optically coupled to each other at a predetermined part (optical coupling portion). The optical component may be provided with a Bragg grating along an advancing direction of light in the optical coupling portion. The optical component according to the present invention includes an optical coupler having a light-multiplexing/demultiplexing function and an optical fiber grating coupler, for example. Such an optical component attains a higher strength and excellent optical coupling characteristics, since optical fibers comprising the structure mentioned above (included in the optical fiber according to the present invention) are employed therein.
In the case where light having a predetermined wavelength entering from one end of the first optical fiber is transmitted through the optical coupling portion without being reflected thereby, so as to be emitted from one end of the second optical fiber, the optical component according to the present invention has excellent optical coupling characteristics to such an extent that the optical power at the one end of the second optical fiber is maintained at 95% of the optical power at the one end of the first optical fiber or more.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.